In a coating station of the curtain coating type a moving support is coated by causing a free falling curtain of coating liquid to impinge onto the moving support to form a layer thereon. An apparatus to perform this method is described in U.S. Pat. No. 3,508,947 to Hughes wherein a multilayer composite of a plurality of distinct layers is formed on a slide hopper and dropped therefrom to form a free falling curtain.
In the curtain coating process, particularly as used to manufacture multilayer photographic materials, the quality of the coating is largely determined by the properties of the liquid curtain. It is important to ensure that a stable laminar liquid sheet is formed by the slide hopper and that an equally stable, laminar flow, liquid curtain is formed from that sheet. To prevent contraction of the falling curtain under the effect of surface tension, it is known that the curtain must be guided at its edges by curtain edge guides.
In general, edge guides are stationary, solid members which are attached to the slide hopper used to supply coating liquid to the curtain and extend downwardly from the initial point of free fall of the curtain, the so called hopper lip. Wetting contact of the edges of the falling curtain with the edge guides should be maintained along the entire length of the edge guides to avoid a break in the curtain.
Curtain stability is commonly defined by the speed at which a disturbance (wave) moves through a curtain relative to the curtain. If the curtain speed is greater than the wavespeed, the disturbance is washed downstream and the curtain is stable. If the curtain speed is less than the wavespeed, then the disturbance propagates upstream and the curtain is unstable. The wavespeed disturbance is given by: EQU C=(2*.delta.*U/(.rho.*Q)).sup.1/2 ( 1)
Where:
C=Wavespeed of a disturbance PA1 .delta.=Local surface tension PA1 U=Local curtain speed PA1 .rho.=Density PA1 Q=Volumetric flowrate per unit width
(See Journal of Colliod and Interface Sciences, Vol. 77, No 2, October 1980, pp. 583-585).
Using the curtain stability criteria that the curtain speed must be greater than the wavespeed: EQU U&gt;C (2)
The curtain stability criterion can be rearranged to read: EQU .rho.*Q*U/2*.delta.&gt;1 (3)
It is clear from equation 3 that the local curtain speed and the volumetric flowrate per unit width, referred to herein as flowrate, are the dominant factors in determining curtain stability, since the density and local surface tension only vary by small amounts.
Equation 3 shows that a lower flowrate yields a less stable curtain. In particular, the flowrate within approximately 0.5 cm of the edge guide typically determines curtain stability, since this region is where the flowrate nonuniformity caused by edging equipment occurs. Therefore, to maximize curtain stability, an edge guide should not adversely affect the thickness uniformity near the curtain edge.
Equation 3 also shows that the curtain stability increases as the local curtain speed increases. Again, the curtain edge typically has the lowest speed since this region experiences the effect of viscous drag forces with the edge wall. Therefore, an edging system which minimizes viscous drag forces will exhibit a more stable curtain.
Lower drag forces have been effectively achieved in the prior art through use of a low viscosity flushing solution to prevent the higher viscosity curtain solution from contacting the edge guide. Another method for reducing the drag forces is to reduce the edge guide-curtain contact area. This has been attempted in the prior art by employing a small diameter edge guide, typically a wire or rod. However, poor curtain stability was observed. Perhaps this is best explained by a force balance analysis at the curtain edge. Surface tension forces attempt to contract the curtain surfaces, and the edge guide is attempting to overcome this contractile force to maintain a constant curtain width. Prior art designs which have been successful have relied upon a relatively large edge guide contact land and the resultant meniscus formed by that land to balance the curtain contractile force. When the contact land width is reduced, an imbalance is observed and the curtain is unstable.
Since edge guides are typically solid members, there is always the possibility of photographic material congealing on the edge guide surface. This is particularly true when there is a stagnant fluid region. Such stagnant fluid regions can occur in the prior art when the curtain moves backward and forward under the influence of pressure differences between the frontside and backside of the curtain. As the curtain moves to and fro, liquid deposits are left on the edge which may eventually congeal. The formation of congealed deposits on the edge guide may act as a disturbance source in the curtain and result in a thickness nonuniformity in the final coating, or produce an unstable curtain.
The prior art is not successful in providing an edge guide which achieves good curtain stability and uniform thickness up to the edge guides, while reducing the propensity to form congealed deposits. Thus, efficient use of the curtain coating method for manufacturing photographic materials has been adversely affected.
The present invention describes a method and apparatus wherein a more stable curtain is formed, the propensity to form congealed deposits is greatly reduced and there is uniform film thickness up to the edge guides.